1. Technical Field
The present disclosure relates to a circuit and a method for dynamic offset compensation in a MEMS sensor device.
2. Description of the Related Art
Known to the art are sensor devices including micromechanical structures made, at least in part, of semiconductor materials and using MEMS (Micro Electro Mechanical Systems) technology.
These sensor devices are integrated in portable electronic apparatuses, such as, for example, portable computers, laptops or ultrabooks, PDAs, tablets, mobile phones, smartphones, digital audio players, photographic or video cameras, and consoles for videogames, affording considerable advantages as regards the occupation of space, in terms of area and thickness.
The sensor devices generally comprise: a micromechanical sensing structure, designed to transduce a mechanical quantity to be detected (for example, a linear or angular acceleration, a pressure, an ensemble of acoustic waves, etc.) into an electrical quantity (for example, a capacitive variation); and an electronic reading circuit, designed to carry out appropriate processing operations (amongst which amplification and filtering operations) of the electrical quantity so as to supply an output electrical signal, either analog (for example, a voltage) or digital (for example, a pulse-density-modulation—PDM-signal).
This electrical signal, possibly further processed by an electronic interface circuit, is then made available for an external electronic system, for example a microprocessor control circuit of an electronic apparatus incorporating the sensor device.
The micromechanical sensing structure in general comprises one or more mobile parts, which are able to undergo deformation or to perform one or more detection movements in the presence of corresponding mechanical quantities to be detected.
In the case of capacitive detection structures, first electrodes are fixedly associated to the mobile part, and set facing second, fixed, electrodes, thereby providing the plates of a detection capacitor element, the capacitance of which is variable as a function of the quantity to be detected.
In a known manner, an offset signal is superimposed on the useful signal at output from the sensor device (i.e., a deviation with respect to the useful signal), with a d.c. frequency contribution, the value of which depends on the manufacturing process used and moreover on the thermal and mechanical stresses to which the sensor device is subject during operation. These stresses are, for example, induced through the package of the sensor device, in particular on account of the different thermal expansion coefficients of the various materials used.
In the worst case, the value of the offset signal may cause the electronic reading circuit to work outside the dynamic range for which it is designed, thus causing errors, for example, saturation thereof, or the impossibility of measuring the mechanical quantities to be detected.
Solutions have thus been proposed for offset compensation and cancelling, which envisage, in particular, trimming of appropriate circuit parameters and electrical elements, for example, variable and trimmable resistors or capacitors, at the end of the manufacturing process of the micromechanical sensing structure.
The above solutions envisage, for example, the use of an offset-compensation structure at input to the electronic reading circuit, designed to generate an unbalancing that is equal and opposite to the one generated by the offset due to the micromechanical sensing structure, in such a way as to compensate, and ideally eliminate, the effects thereof. For example, in the case of a micromechanical sensing structure of a capacitive type, this unbalancing may be constituted by a variation of charge injected at input to the electronic reading circuit.
These solutions do not enable, however, compensation of a variation in time of the offset, which occurs during use of the sensor device, for example on account of the possible thermal and mechanical stresses to which it is subjected.
In this regard, the ever-increasing demand for a reduction in the dimensions of MEMS sensor devices entails a corresponding reduction of the value of the electrical quantities detected (which may, for example, reach values in the region of some tens of attofarads, aF, in the case of capacitive sensing structures).
Consequently, the aforesaid offset variations may be comparable to, if not even higher than, the useful electrical signal, thus making it difficult, if not unfeasible, the execution of measurements and detection operations.
In other words, the offset issue is increasingly more critical as the dimensions of the MEMS sensor devices decrease, given that the size reduction entails an increase in the sensitivity to stresses and a decrease in the transduction gain and hence in the detected electrical signal.